Dr. Karl Shuker writes a great blog, ShukerNature. His biggest-ever "hit" with readers wasn't about new species or lake monsters. It was about photos on the internet about a coal-black lion. A lot of people seem to have seen the photo but missed the description. Someone uploaded to the Net a photoshopped version of a real lion photograph.
Alas, no one has a black lion, or photographic evidence of one. Nonetheless, there are eyewitness claims that such a dramatic color variant exists. It wouldn't be surprising if melanism occurred on rare occasions. It does in several cat species. A raging debate exists over whether a truly black puma has ever existed, although there are a few photos of dark-looking examples, and my dad saw a black-looking one in Maine in the 1950s.
The most extreme example of an off-colored cat may be the blue tigers once reported from China. A blue has never ended up in a zoo or on film, but the color does, on extremely rare occasions, occur in lynx.
So people should keep an eye out for the legendary black lion. You know how sneaky cats can be....